1. Field of the Invention
Example embodiments of the present invention relate generally to communications systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Conventional wireless communication systems include one or more base stations or base station routers, which may also be referred to as access points, node-Bs or access networks, for providing wireless connectivity to one or more mobile units, which may also be referred to using terms such as user equipment, subscriber equipment, and access terminals. Examples of mobile units include cellular telephones, personal data assistants, smart phones, text messaging devices, laptop/notebook computers, desktop computers and the like. Each base station may provide wireless connectivity to one or more mobile units, such as the mobile units in a geographical area, or cell, associated with the base station. Alternatively, a base station router may be used to provide wireless connectivity to the mobile units.
Messaging sent from a base station or base station router to one or more mobile units is generally referred to as “forward link” or “downlink” messaging. Messaging sent from a mobile unit to a base station or base station router is generally referred to as “reverse link” or “uplink” messaging.
Orthogonal frequency division multiplexing (OFDM) is an efficient modulation scheme for signal transmission over frequency-selective channels. In OFDM, a wide bandwidth is divided into multiple narrow-band sub-carriers, which are arranged to be orthogonal with each other. The signals modulated on the sub-carriers are transmitted in parallel.
OFDM may be used to support multiple access for multiple subscribers through time division multiple access (TDMA), in which each subscriber uses all the sub-carriers within its assigned time slots. Orthogonal frequency division multiple access (OFDMA) is another method for multiple access, using the basic format of OFDM. In OFDMA, multiple subscribers simultaneously use different sub-carriers, in a fashion similar to frequency division multiple access (FDMA) (e.g., for each “shared” sub-carrier, frequency divisions are used to allow multiple access).
OFDMA divides a signal into sub-channels (i.e., groups of carriers), with each sub-channel being allocated to a different subscriber. Different sub-channels may then be combined from various carriers. Each subscriber can be treated separately, independent of location, distance from the base station, interference and power requirements. Various modulations can be used for each of the carriers in the system to provide improved coverage and throughput. The sub-channel structure of the OFDMA enhancement enables more efficient duplexing techniques, such as Frequency Division Duplexing (FDD) and Time Division Duplexing (TDD), and creates a signal with reduced interference and capable of higher data throughput. In FDD systems, both forward link and reverse link transmissions occur simultaneously on different carriers.
FIG. 1 illustrates a conventional OFDMA system 100. As shown in FIG. 1, the OFDMA system 100 includes a plurality of user equipments (UEs) 105 in communication with one or more serving Node Bs 120/125 over an air interface. The plurality of Node Bs are connected to a radio network controller (RNC) 130 with a wired interface. Alternatively, while not shown in FIG. 1, the functionality of both the RNC 130 and Node Bs 120/125 (alternatively referred to as “base stations”) may be collapsed into a single entity referred to as a “base station router”. The RNC 130 accesses an internet 160 through a gateway support node (GSN) 150 and/or accesses a public switched telephone network (PSTN) 170 through a mobile switching center (MSC) 140.
FIG. 2 illustrates a transmitter 200 in a conventional OFDMA system. In an example, the transmitter 200 of FIG. 2 is employed within a transmitting entity (e.g., Node B 120/125, UE 105/110, etc.) within the conventional OFDMA system 100 of FIG. 1.
As shown in FIG. 2, the transmitter 200 includes a modulator 210, a serial-to-parallel (S2P) converter 220, an Inverse Fast Fourier Transformer (IFFT) module 230, a cyclic prefix inserter 240 and a time domain filter 250. The IFFT module 230 includes N ports for receiving modulation symbols. Each of the N ports is associated with an orthogonal sub-carrier. IFFT module 230 is operable to use an N×N IFFT matrix to perform transform operations on its inputs, wherein the entries of the matrix Fj,k are defined as Fj,k=e−2πijk/n, j, k=0, 1, 2, . . . , n−1 and i=√{square root over (−1)}.
Encoded data symbols are provided as input to modulator 210. Modulator 210 uses well-known modulation techniques, such as BPSK, QPSK, 8PSK, 16QAM and 64QAM, to convert the encoded data symbols into K modulation symbols Sk which are then provided as input to S2P converter 120, where K≦N. The S2P converter 220 outputs parallel streams of modulation symbols, which are provided as inputs to one or more of the N ports of the IFFT module 230 associated with orthogonal sub-carriers over which the encoded data symbols are to be transmitted. In the IFFT module 230, an inverse fast Fourier transformation is applied to the modulation symbols Sk to produce a block of chips cn, where n=0, . . . , N−1. The cyclic prefix inserter 240 copies the last Ncp chips of the block of N chips and prepends them to the block of N chips producing a prepended block. The prepended set is then filtered through time domain filter 250 and subsequently modulated onto a carrier before being transmitted.
OFDMA systems provide reduced interference and higher data rates on the reverse link, as compared to conventional Code Division Multiple Access (CDMA), due to the in-cell orthogonality property of OFDMA transmissions. However, OFDMA comes at the cost of increased signaling and a failure to provide bandwidth sharing for users transmitting at lower data rates and/or requiring fast access to a carrier. In contrast, CDMA systems allow multiple subscriber access without explicit request-and-grant mechanisms, as present in conventional OFDMA, and this may increase user access to the carrier for transmissions.
Power control is a critical problem for the reverse link in CDMA systems because CDMA systems may experience significant in-cell and outer-cell interference. OFDMA systems typically experience less in-cell interference than CDMA systems due to the orthogonality property of OFDMA, and thereby OFDMA systems may employ “looser” power control requirements since interference present in an OFDMA system may be substantially limited to outer-cell interference. However, reverse link power control in OFDMA systems remains a problem in conventional OFDMA systems not withstanding the lesser in-cell interference. For example, effective rate control on the reverse link in OFDMA systems may be difficult to achieve without a reverse link transmission power to reverse link data rate mapping.
Since OFDMA transmissions are scheduled by the base-station by giving different portions of available bandwidth to different users, the transmissions per user are typically “bursty” by nature. Accordingly, it is inefficient to maintain a constant pilot transmitted by all users for performing closed loop OFDMA power control. On the other hand, purely open loop power control techniques are limited in their efficiency since open loop power control techniques typically do not maintain a tight control of outer-cell interference, and a prediction of a received signal-to-interference+noise ratio (SINR) for a given transmit power is less accurate. Accordingly, because OFDMA systems do not transmit continuous pilot signal transmissions, it is more difficult to control reverse link transmission power at all users within the cell because reverse link power control must be performed individually for each user.